The present invention relates to fluid motors such as pneumatic cylinders and, more particularly, to a cylinder having a unique end cap construction and novel means for attaching the end caps to the tube.
A popular design of commercial cylinders presently in use includes a hollow tube which is closed at the ends by end caps which are connected together by external bolts to form a closed tie-rod cylinder. One of the end caps is formed with a bore to receive the piston rod. Porting for the cylinder is provided in the end caps.
This tie-rod cylinder design has presented a variety of operational and manufacturing limitations and problems. For example, the provision of an effective and durable seal between the piston and interior of the tube is of great importance. However, since the end caps of such cylinders are rigidly attached to the tube, a relatively small amount of misalignment between the piston rod and the tube may cause the piston to bind against the tube wall. To avoid this problem, such cylinders are designed with a relatively large clearance between the piston and the tube. It has been found that this clearance creates wobbling or vibration which greatly increases the difficulty of obtaining an effective seal between the piston and tube. Such wobbling movement and vibration tend to accelerate the failure of the piston seal by inducing rolling, extrusion and loss of shape retention in the seals. In addition, because many known cylinders, including the tie-rod cylinders described above, are crimped or swaged together when made, it is often expensive or impossible to repair such seal failures. It is desirable to provide a cylinder which will accommodate axial misalignment between the piston rod and the tube without the necessity of providing a large clearance between the piston and the tube.
Most prior art pneumatic cylinders require lubricant between the piston seal and inside surface of the tube to lower the coefficient of friction therebetween for smooth reciprocation of the piston within the tube and to achieve acceptable cycle life of the piston seal. Whether the pistons are pre-lubricated or provided with a lubricant reservoir, it is often difficult to initiate piston movement where the cylinder has not been used for some time, such as overnight. This occurs because of the natural resiliency or memory of the rubber used to make the piston seals. It has been found that when operation of the cylinder ceases, any lubricant then between the piston seal and tube interior wall is pushed out by the resilient force or memory of the rubber. Consequently, the piston seal rests against the interior surface of the tube without lubricant therebetween. When use of the cylinder is resumed, a relatively large force is require to overcome static friction between the unlubricated portion of the tube surface and the piston seal disposed thereagainst. Consequently, it is desirable to provide a piston and cylinder which do not require lubrication and do not have a large amount of start-up friction.
Some cylinders use end blocks which extend beyond the circumference of the cylinder tubes. These present a mounting problem because of their bulkiness. It is desirable to provide a cylinder which is compact and provides versatile mounting means.